It is a Boss O-ring fitting, #4 or 6 if I rememeber correctly. Just get a Boss to NPT conversion then you can hook up an oil pressure gauge, hard piped in behind the oil filter housing, neat and tidy, but took some time as it is hard to get into.
thanks guys, yeah theres not much room for an t fitting or anything, might just end up buying that oil guard filter cap, then just running my gauge off that, maybe put a quick connect for the electric so when i chg filter i just unplug and then plug back in, not sure